The present invention relates to a wet type electrophotographic copying machine and more particularly to a wet type electrophotographic copying machine provided with a mechanism capable of making a copy during substantially two revolutions of a photoconductor.
Referring to FIG. 1, a conventional electrophotographic copying machine of the type described will now be explained. In the figure, around a drum-shaped photoconductor (hereinafter referred to as the photoconductor drum), there are arranged in order (in the direction indicated by the arrow R) a charger 2 for charging and image transfer, an exposure apparatus 3, a quenching charger 4, a development section 5, a cleaning member 6 and a squeegee roller 7. The cleaning member 6 is rotatable about a shaft 8 and, by the rotation thereof, a top edge portion of the cleaning member 6 can be brought into contact with the surface of the photoconductor drum 1 is detached therefrom. The squeegee roller 7 is disposed in close proximity to the surface of the photoconductor drum 1 with a small gap G and is rotatable in the direction opposite the rotating direction (indicated by the arrow R) of the photoconductor drum 1. A squeegee blade 9 is in contact with the peripheral surface of the squeegee roller 7.
The operation of the copying machine will now be explained.
When the main switch (not shown) of the copying machine is turned on, the photoconductor drum 1 begins to rotate and a liquid developer pump (not shown) for supplying a liquid developer 10 to the development section 5 is actuated. During the first revolution of the photoconductor drum 1, the surface of the photoconductor drum 1 is uniformly charged by corona charges applied by the charger 2. An optical image of an original is projected onto the surface of the charged surface of the photoconductor drum 1 by the exposure apparatus 3 to form an electrostatic latent image on the surface of the photoconductor drum 1. The electrostatic latent image bearing surface of the photoconductor drum 1 passes over the deenergized quenching charger 4 and comes to the development section 5, so that the liquid developer 10 is deposited on the surface of the photoconductor drum 1 and the electrostatic latent image is developed to a visible image.
Just before the leading edge of the thus formed image on the photoconductor drum 1 comes to the cleaning member 6, the cleaning member 6 is disengaged from the surface of the photoconductor drum 1. The position on the surface of the photoconductor drum 1 from which the cleaning member 6 has been disengaged comes to the position of the squeegee roller 7, excess developer is removed from the surface of the photoconductor drum 1, and the developer removed from the surface of the photoconductor drum 1 is recovered into a developer tank through a squeegee blade 9.
The photoconductor drum 1 successively makes the second revolution and, at the same time, an image transfer sheet (not shown) is fed from a sheet feeding apparatus (not shown) between the photoconductor drum 1 and the charger 2, and the developer deposited on the surface of the photoconductor drum 1 is transferred to the transfer sheet. The transfer sheet is then subjected to image fixing by drying and discharged from the copying machine. After the image transfer, the photoconductor drum 1 is rotated and the surface of the photoconductor drum 1 passes under the exposure apparatus 3 from which no light is projected, and the developer remaining on the surface of the photoconductor drum 1 is removed by the cleaning member 6 which is brought back into contact with the surface of the photoconductor drum 1, so that one copying cycle is completed and the electrophotographic copying machine is ready for the next copying.
The advantages of a wet type of electrophotographic copying machine of the type in which one copy is made during two revolutions of a photoconductor drum, over an ordinary wet type electrophotographic machine of the type capable of making one copy during one revolution of a photoconductor drum, are as follows:
The first advantage is that, since the cleaning member 6 can be disposed near the development section 5, the liquid developer in the development section 5 can be used as the cleaning liquid for cleaning the surface of the photoconductor drum 1.
The second advantage is that, since the cleaning member 6 is disposed near the development section 5, it is easy to recover the liquid developer which has been used as the cleaning liquid.
The third advantage is that, since the charger 2 can serve as a corona charger for applying charges to the surface of the photoconductor 1 and as a charger for transferring the developed image from the surface of the photoconductor drum 1 to the transfer sheet, the number of the required devices can be reduced in comparison with the ordinary electrophotographic copying machine.
In the electrophotographic copying machine provided with the mechanism for making one copy during two revolutions of the photoconductor drum 1, the gap G between the photoconductor drum 1 and the squeegee roller 7 has a significant effect on the copying performance of the copying machine. Specifically, when the gap G is great, the squeezing of the liquid developer cannot be initiated properly. As a result, more developer tends to be deposited in the leading edge portion of the image formation area on the surface of the photoconductor drum 1 than in the remainder thereof. The amount of the developer deposited in the leading edge portion is such that it flows down along the surface of the photoconductor drum 1. Furthermore, when the leading edge portion of the photoconductor drum 1 comes near the charger 2 (which has been energized to apply charges to the surface of the photoconductor drum 1 for attaining a stable image transfer effect, before the image transfer sheet comes to the charger 2), the liquid developer is electrically attracted in the downstream direction with respect to the rotation of the photoconductor drum 1, so that the flow of the liquid developer along the surface of the photoconductor drum 1 is accelerated, and excess developer is deposited in the leading edge portion of the image transfer sheet.
In order to avoid such a problem, it is conceivable to lengthen the distance between the squeezing initiation position and the image formation area on the photoconductor drum 1, thereby avoiding the deposition of the liquid developer on the image transfer sheet even if the developer flows down as mentioned above. However, the size of the image formation area on the photoconductor drum 1 is determined by the maximum copying size of the copying machine. Therefore, it is required that the size of the photoconductor drum 1 be increased in order to lengthen the distance between the squeezing initiation position and the image formation area on the photoconductor drum 1, with the result that the copying machine has to be increased in size. Furthermore, in order to attain a predetermined copying speed, the peripheral speed of the photoconductor drum 1 has to be increased corresponding to the increased length between the squeezing initiation position and the image formation area on the photoconductor drum 1.
On the other hand, if the gap G between the photoconductor drum 1 and the squeegee roller 7 is decreased as much as possible, the squeezing of the developer can be done effectively even at the squeezing initiation position. However, the amount of the developer deposited on the photoconductor drum 1 is so decreased that the image density of the copy may be lowered. If the gap G between the photoconductor drum 1 and the squeegee roller 7 varies, it may occur that the photoconductor drum 1 comes into complete contact with the squeegee roller 7. Therefore, the positioning of the photoconductor drum 1 and the squeegee roller 7 has to be controlled most accurately, but it decreases the efficiency of the production of the copying machine and becomes costly.